(1) Field of the Invention
This invention is directed to helicopter rotor vibration reduction; in particular to the placement of removable self-adhering elastomer ramp profiles upon the trailing edge of the rotor blades for the purpose of reducing dynamic vibration. It is also directed to improving rotor blade track characteristics, that is, the out-of-plane displacement of the blade tips to ensure that they all spin in the same rotational plane.
(2) Description of Related Art
Others have attempted to control the dynamic vibrations of rotor blades through the attachment of weights and trim tabs on the blades. For example, U.S. Pat. Nos. 2,430,948, 3,237,697, and 4,789,305 all show the use of weights or trim tabs that are added to the blades. Weights may be relocated to a new position on the rotor based on test measurements to reduce vibration. Trim tabs may be bent to alter the aerodynamic pitching moment at specific location on the rotor blade to improve rotor track and to reduce vibration.
Others have used dynamic balancing of the rotors either in a laboratory setting or in an actual flight test. For example, U.S. Pat. No. 7,097,427 discloses the use of a whirl test stand or the use of the helicopter and U.S. Pat. No. 2,430,948 discloses the use of a laboratory test on a spindle or upon the aircraft itself. U.S. Pat. No. 5,239,468 describes an automated in-flight helicopter maintenance monitoring system for collecting airframe and component maintenance data along with rotor balance and tracking data. However, without a rapid method to make a quick adjustment to the rotor blade, changes to alter rotor vibration and improve rotor track characteristics still require significant time and the use of specialized equipment.
Others have tried various bolt-on style weight mechanisms to correct blade induced airframe vibrations. For example, U.S. Pat. No. 5,273,398 is a washer weight system that is attached through a stud and locking nut. This system is less attractive as it requires significant disassembly and is positioned at only one location on the blade. It is less adjustable as it does not provide for a more continuous adjustment by the use of many weights over the length of the blade.
Others have permanently bonded ramp tabs to the blades based on an initial vibration test. For example, U.S. Pat. No. 3,952,601 describes a method of balancing blades initially and then permanently bonding the ramp tabs to the blade. This method has important practical difficulties. Once installed, the ramp tabs are difficult to remove since the permanent epoxy adhesive used to install these ramp tabs has significant strength which requires the use of tools to ply or scrape the tab off the surface of the blade. As a result, the removal of the ramp tab after bonding requires the destructive removal of the tabs or paint on the blade surface. Consequently, whenever there is a vibration change in the blade over the service life, tab removal damages the blade. The epoxy adhesive requires a significant level of effort to be used in any removal method. In most cases, the rotor blade needs to be removed from the helicopter to provide proper access to perform the removal, adhesive cleanup, and repair of the blade surface. This degree of repair adds significant risk of damage to the structure of the blade substrate. Reinstallation of the ramp tabs in a field environment with the use of permanent epoxy adhesive is also not practical. The process of rotor vibration smoothing and track adjustment is often iterative. The method of adhesive application and curing associated with epoxy bonding of ramp tabs is not advantageous to the timely completion of the rotor smoothing process.
Additionally, the method described in U.S. Pat. No. 3,952,601 is a whirling stand method that compares the rotor blade to a master blade rather than an in flight measurement. It is not a practical iterative field method and only results in the blade being adjusted to match the master blade at a limited operational condition that does not include translational velocities of the helicopter.
A permanently bonded ramp tab is not desirable due to changes that occur over the service life of a rotor blade. The changes include wear, moisture absorption, and blade shape changes that all helicopter rotor blades experience over their useful lives. These changes can degrade tracking and vibration characteristic of the rotor and cause vibration changes in the airframe.
Also, it has been found that the use of a whirling stand method for rotor smoothing as described in U.S. Pat. No. 3,952,601 is not optimum for achieving both blade tracking and airframe vibration corrections over the full flight speed range in actual use. What works well at one particular speed may need to be optimized at a range of speeds. Stationary whirl stands can only replicate the rotor operating conditions of vertical thrust and rotational speed, but not forward flight speed. Achieving satisfactory rotor vibration levels for any translational flight speed is not possible with the use of only a stationary whirling stand method. As a result, additional adjustments are required to achieve acceptable blade tracking and airframe vibration over the full flight speed range in actual use.
The inability to make field changes has made permanently bonded ramp tabs impractical in actual use as a primary method for periodic blade tracking and vibration reduction. As a result, permanently bonded ramp tabs are not widely used in the helicopter industry.
Currently, field adjustments to helicopter rotor tracking and vibration are performed by a trim tab method for the majority of all helicopter models and manufacturers. The trim tab method consists of a narrow metal flat panel that is permanently bonded to the helicopter rotor blade and overhangs the trailing edge. It is bent upward or downward by using a special tool, typically at an angular change of 1 to 10 degrees based on field vibration measurements. The trim tab is the current state of the art for field adjustments to correct rotor tracking and vibration and is practical for use in a field environment. Unfortunately, this method is imprecise and normally requires numerous iterations to achieve the desire result. Repeatability of the angular change is often difficult since the magnitude of the required angle change of the trim tab is often very small. Another shortcoming of the trim tab method is that the metal tab itself will move over time, resulting in degradation to rotor track and vibration. Also, trim tab positions will move as a result of blade flexing during flight. The result of this phenomenon is the necessity to re-adjust the angular position of the trim tabs on a frequent basis to maintain low vibration levels.